Search

Fe‐Mn based layered oxides are recognized as promising cathode materials for sodium‐ion batteries (SIBs) with high capacities and earth‐abundant ingredients. However, their real‐world applications are still constrained by fast capacity decay accompanied with the requirements of deeper insights into the principles behind. Herein, taking O3‐NaxFe1/2Mn1/2O2 as a classic sample, the capacity fading mechanism of Fe‐Mn based layered oxides is comprehensively investigated through combined techniques. For the first time, it is revealed that Fe migration is merely triggered after the oxidation of ≈0.3 mol Fe3+ based on solid proofs from ex situ X‐ray absorption spectroscopy and Mössbauer spectroscopy, which implies the crucial role of the accumulated structural distortion induced by Jahn–Teller active Fe4+. O3‐P3 phase transition during cycling is obviously constrained along with Fe migration as evidenced by in situ/ex situ X‐ray diffraction, well interpreting the intensified polarization and the resulting large capacity loss. More importantly, within the desodiation depth (≈80% of sodium extraction) where Fe migration is almost absent, the capacity fading is dominantly rooted in the Fe4+ activated and Mn‐dissolution aggravated surface passivation as confirmed by mass/X‐ray spectroscopies and electrochemical analysis. These renewed understandings of the fast capacity decay in Fe‐Mn based layered oxides offer clearer clues for designing desirable cathodes for SIBs. [ABSTRACT FROM AUTHOR]

Sodium‐ion batteries (SIBs) have grasped renewed attentions in recent years owing to the blooming growth of clean energy and corresponding demands of grid‐scale energy storage. Note that, the properties of cathode materials are quite important to the development of SIBs as they largely determine the energy density, cycle life, and cost of a battery. Direct inheriting those Co/Ni based layered cathodes that are successfully utilized in lithium‐ion batteries seems to be impracticable for SIBs in view of the high materials costs. Fortunately, the discovery of the electrochemical activity of Fe3+/Fe4+ redox couple in sodium layered materials has opened a new way to design high capacity/voltage and low‐cost cathodes for SIBs. To date, various Fe‐based layered oxides have been explored, showing encouraging electrochemical performances in SIBs. However, issues and challenges still remain upon the practical utilization of Fe‐based layered cathodes, calling for further targeted and in‐depth investigations. In this review, the developing history, key problems, and current progress of Fe‐based layered cathodes on electrochemical performances and working mechanism are summarized and discussed. In addition, possible research trends and perspectives are proposed. It is believed that Fe‐based layered oxides will be one of the most attractive cathodes for SIBs. [ABSTRACT FROM AUTHOR]

Ni/Mn-based oxide cathode materials have drawn great attention due to their high discharge voltage and large capacity, but structural instability at high potential causes rapid capacity decay. How to moderate the capacity loss while maintaining the advantages of high discharge voltage remains challenging. Herein, the replacement of Mn ions by Ga ions is proposed in the P2-Na 2/3 Ni 0.2 Mn 0.8 O 2 cathode for improving their cycling performances without sacrificing the high discharge voltage. With the introduction of Ga ions, the relative movement between the transition metal ions is restricted and more Na ions are retained in the lattice at high voltage, leading to an enhanced redox activity of Ni ions, validated by ex situ synchrotron X-ray absorption spectrum and X-ray photoelectron spectroscopy. Additionally, the P2-O2 phase transition is replaced by a P2-OP4 phase transition with a smaller volume change, reducing the lattice strain in the c-axis direction, as detected by operando/ex situ X-ray diffraction. Consequently, the Na 2/3 Ni 0.21 Mn 0.74 Ga 0.05 O 2 electrode exhibits a high discharge voltage close to that of the undoped materials, while increasing voltage retention from 79% to 93% after 50 cycles. This work offers a new avenue for designing high-energy density Ni/Mn-based oxide cathodes for sodium-ion batteries., (© 2023 Wiley‐VCH GmbH.)